title: Gunshot induced indirect femoral fracture: mechanism of injury and fracture morphology.
abstract: BACKGROUND
  Indirect ballistic fractures occur when a projectile passes close to, but not contacting, the bone. The mechanism of how these fractures occur is not yet proven, but recently the acoustic shockwave has been excluded as a cause. The objective of this study is to determine whether the expanding temporary cavity, the collapse of this cavity or its oscillation causes these fractures. In addition, we describe the fracture morphology and biomechanical causes of this injury.
  
  
  METHODS
  40 fresh deer femora were strain gauged and embedded in ballistic gelatin before being shot with four different projectiles with varying distances off the bone. Pressure recordings, chronographs and radar allowed assessment of local pressures and energy transfer. High-speed video allowed the temporal relationship between the temporary cavity and fracture formation to be analysed, while sample dissection allowed the fracture morphology to be described.
  
  
  RESULTS
  The fractures produced were consistently wedge-shaped and caused by the expansion of the temporary cavity, flexing the bone beyond its yield point, causing tension failure on the cortex opposite the expanding temporary cavity and a compression wedge on the side of the cavity. Local pressure was not predictive of fracture formation but the energy transfer to the gelatin block was predictive.
  
  
  CONCLUSIONS
  Indirect fractures are caused by the expansion of the temporary cavity and relate to the proximity of this cavity to the bone. Fractures occur from flexion of the bone and classically display wedge-shaped fracture patterns with the apex of the wedge pointing away from the expanding cavity.
authors.affiliation: Surgical Sciences, Orthopaedic Surgery, Health Sciences, Dunedin School of Medicine, University of Otago, Dunedin, Otago, New Zealand and the New Zealand Defence Force.
authors.forename: David C
authors.initials: DC
authors.lastname: Kieser
authors.forename: D J
authors.initials: DJ
authors.lastname: Carr
authors.forename: S C J
authors.initials: SC
authors.lastname: Leclair
authors.forename: I
authors.initials: I
authors.lastname: Horsfall
authors.forename: J C
authors.initials: JC
authors.lastname: Theis
authors.forename: M V
authors.initials: MV
authors.lastname: Swain
authors.forename: J A
authors.initials: JA
authors.lastname: Kieser
mesh_terms: D004735:Energy Transfer; D005264:Femoral Fractures; D005269:Femur; D011312:Pressure; D014948:Wounds, Gunshot
keywords: Accident & Emergency Medicine; Orthopaedic & Trauma Surgery; Radiology & Imaging